The dispatching of production lots in a prioritized and scheduled fashion provides for smooth and reliable operation of a manufacturing system. Shop floor scheduling systems may be used to decide the sequence of a set of queued lots to dispatch to a given tool in the manufacturing line. For simple manufacturing systems, a first-in-first-out (FIFO) rule is often a reasonable choice. However, in semiconductor manufacturing, there are various tool-specific and lot-specific scheduling attributes which make this a poor or infeasible choice.
Semiconductor manufacturing utilizes a variety of tools and steps, and a production lot typically undergoes a large number of discrete processing steps during manufacture. It is common for different production lots to be simultaneously processed through a manufacturing line (e.g., for different products and/or different customers), which requires sharing of tools amongst the various lots. These complexities of the manufacturing process and the pressures exhibited by the customers (e.g., delivery times, reducing lead time, etc.) require that some lots be split into separate smaller lots to improve the speed of the lot movement through the manufacturing line. This splitting of lots allows the supply chain to better meet the delivery requirements, e.g., by permitting incremental deliveries to the customer, but also causes the individual units of an original single production lot to become spread out over the length of the manufacturing line.
In semiconductor manufacturing, the quality of manufactured units (e.g., integrated circuit chips) can be highly dependent on the silicon wafer in which the units are built. In light of this, and for quality tracking and control, some customers request that delivered units be built from a same lot and/or that lots of end product assemblies be made of same component lots. However, the splitting of a single lot into plural sub-lots throughout the manufacturing line can affect the delivery of a group of units, since large gaps can form between related sub-lots in the manufacturing line.